The most effective and most recommended method of removing ink from a leather seat is to seek out the help of a professional. If this isn't an option, however, there are several things that may do the job if done with care.
The key to removing ink from a leather car seat is to work as quickly as possible after the ink has spilled. It is important to use whatever is available, preferably a clean paper towel or the like, to remove the spilled ink before it spreads or sets in. A bit of soap that contains moisturizer combined with water can help to remove the remainder of the ink. A commercial grade leather cleaner may also be helpful.
If these options fail, using a small amount of alcohol on the stain or hairspray and then removing the excess immediately might help. These should be dried completely and quickly in order to keep the alcohol from deteriorating the leather. Repeated attempts should be made, being very careful not to wear through the leather. In some cases, it is not possible to remove the ink stain from the leather, especially when it has been left too long or left in the heat of the day to meld with the leather.